Ants are friendly to some trees, but not others

Tree-dwelling ants generally live in harmony with their arboreal hosts. But new research suggests that when they run out of space in their trees of choice, the ants can get destructive to neighbouring trees.

The research, published in the November issue of the American Naturalist, is the first to document that ants bore into live trees, and it reopens a centuries-old debate on the relationship between ants and plants.

Ants and certain species of plants and trees have cosy relationships. Myrmecophytes, also knows as ant-plants, have hollow stems or roots that occur as a normal part of their development. Ant colonies often take residence in these hollows. To protect their homes, the ants patrol the area around the tree, killing insects that want to eat the plant's leaves and sometimes destroying vegetation of other plants that might compete for precious soil nutrients and sunlight. The relationship is a classic biological mutualism. The ants get a nice place to live; the trees get protection. Everybody wins.

But while researching ant-plants in the Amazonian rainforests of Peru, Douglas Yu of the University of East Anglia and Glenn Shepard of Sao Paulo University were tipped off by the local people about a strange phenomenon. The natives showed the researchers several non-myrmecophyte trees with swollen scars called galls on their trunks and branches. When the researchers cut into the galls, they found that ants had excavated tunnels into the live wood.

"Ants are superb ecosystem engineers," David Edwards the lead author of the study said, "but this is the first example of ants galling trees to make housing."

Megan Frederickson, a Harvard biologist and member of the research team, searched 1,000 square kilometres of forest and found numerous galled trees inhabited by ants, suggesting the behaviour is not uncommon. The galled trees were only found on the edges of "Devil's gardens" – ant-made forest clearings that surround stands of ant-plants. It appears, the researchers say, that when the colonies fill the available space in the ant-plants, they branch out and carve new nests into neighbouring trees.

The discovery reopens a debate that raged among Charles Darwin and his contemporaries about the relationship between ants and plants. Darwin believed – rightly as it turned out – the hollow spaces in ant-plants occurred as part of the plant's normal development. Since the ants did no damage to the plant, the relationship could be considered a mutualism. Botanist Richard Spruce disagreed. He believed the ants bored the hollows themselves and that the trees needed ants "like a dog needs fleas." In Spruce's view, ants are parasites.

Studies in the 1960s showed definitively that ant-plant hollows occur normally, vindicating Darwin. But this latest finding that ants do gall non-myrmecophytic trees shows that Spruce wasn't so wrong after all.

Orphan army ants join nearby colonies

Army ants are nomadic and colonies emigrate frequently. Here, two soldiers of the army ant Dorylus molestus guard a colony emigration at Mt Kenya. Photo: Daniel Kronauer/Harvard UniversityColonies of army ants, whose long columns and marauding habits are the stuff of natural-history legend, are usually antagonistic to each other, attacking soldiers from rival colonies in border disputes that keep the colonies separate. But new work by a researcher at the Harvard Museum of Comparative Zoology and colleagues at the University of Copenhagen shows that in some cases the colonies can be cooperative instead of combative.

In those cases, when an army ant colony loses its queen, its workers are absorbed, not killed, by neighbouring colonies, and within days are treated as part of the family.

The research, conducted in an ant-rich area on the slopes of Mount Kenya, is detailed in the journal Proceedings of the Royal Society B.

Army ant colonies are dominated by a single, large queen who produces the eggs that give rise to all of the colony's individuals, which can number millions of workers. When she dies, colonies quickly disappear, raising the question of what happens to the many individuals.

The work was conducted by Daniel Kronauer, a junior fellow in Harvard's Society of Fellows, over two field seasons in Kenya. Kronauer and his colleagues followed the fates of 10 army ant colonies whose queens they had removed. The researchers lost track of two of the colonies but observed two distinct strategies used by the remaining eight.

Most of the queenless colonies, seven out of 10, simply joined a neighbouring colony – determined by genetic analysis – with the new workers slowly losing their distinct colony odour, and within days becoming fully integrated.

The last queenless colony stayed on its own, with workers employing a strategy of producing a small brood of winged males. Though these males were removed for analysis, in an undisturbed colony the males would fly off looking for young unmated queens. Though this strategy does provide some chance of passing along the colony's genes, the small number of males produced – just 31 in this case compared with 3,000 in a fully functioning colony – illustrates that this strategy may not be efficient.

"It is a last option to get some fitness returns before they die," Kronauer said.
Though the work illuminates where most of the ants go, it doesn't entirely settle the question of why. Though it makes sense that the accepting colony would want to increase its size and competitiveness by gaining many new workers, the evolutionary drivers behind the queenless colony's workers absorbing into the new colony are vaguer.

Since workers are normally kept from reproducing, Kronauer and his co-authors found, there doesn't seem to be a direct fitness benefit. Instead, the authors suggested that the fusing colonies are driven by indirect fitness benefits gained because of distant relationships between colonies, and they called for more fieldwork to explore the issue further.

Ants more rational than humans

Ants are more rational collective decision makers than humans.
Photo: Stephen Pratt/Arizona State University

In a study released online on July 22 in the journal Proceedings of the Royal Society: Biological Sciences, researchers at Arizona State University and Princeton University show that ants can accomplish a task more rationally than our – multimodal, egg-headed, tool-using, bipedal, opposing-thumbed – selves.

This is not the case of humans being "stupider" than ants. Humans and animals simply often make irrational choices when faced with very challenging decisions, note the study's architects Stephen Pratt and Susan Edwards.

"This paradoxical outcome is based on apparent constraint: most individual ants know of only a single option, and the colony's collective choice self-organizes from interactions among many poorly-informed ants," says Pratt, an assistant professor in the School of Life Sciences in ASU's College of Liberal Arts and Sciences.

The authors' insights arose from an examination of the process of nest selection in the ant, Temnothorax curvispinosus. These ant colonies live in small cavities, as small as an acorn, and are skilful in finding new places to roost. The challenge before the colony was to "choose" a nest, when offered two options with very similar advantages.

What the authors found is that in collective decision-making in ants, the lack of individual options translated into more accurate outcomes by minimizing the chances for individuals to make mistakes. A "wisdom of crowds" approach emerges, Pratt believes.

"Rationality in this case should be thought of as meaning that a decision-maker, who is trying to maximize something, should simply be consistent in its preferences." Pratt says. "For animals trying to maximize their fitness, for example, they should always rank options, whether these are food sources, mates, or nest sites, according to their fitness contribution."

"Which means that it would be irrational to prefer choice 'A' to 'B' on Tuesday and then to prefer 'B' to 'A' on Wednesday, if the fitness returns of the two options have not changed."

"Typically we think having many individual options, strategies and approaches are beneficial," Pratt adds, "but irrational errors are more likely to arise when individuals make direct comparisons among options."

Studies of how or why irrationality arises can give insight into cognitive mechanisms and constraints, as well as how collective decision making occurs. Insights such as Pratt's and Edward's could also translate into new approaches in the development of artificial intelligence.

"A key idea in collective robotics is that the individual robots can be relatively simple and unsophisticated, but you can still get a complex, intelligent result out of the whole group," says Pratt. "The ability to function without complex central control is really desirable in an artificial system and the idea that limitations at the individual level can actually help at the group level is potentially very useful." Pratt is a member of Heterogeneous Unmanned Networked Team (HUNT), a project funded by the Office of Naval Research (ONR) to enable to development of bio-inspired solutions to engineering problems.

What do these findings potentially say about understanding human social systems? "It is hard to say. But it's at least worth entertaining the possibility that some strategic limitation on individual knowledge could improve the performance of a large and complex group that is trying to accomplish something collectively," Pratt says.